In a magnet system for MRI imaging comprising a superconducting magnet (10) mounted within a cryogen vessel (12), apparatus is provided for controlling egress of cryogen gas from the cryogen vessel. The apparatus comprises a controlled valve (42) linking the interior of the cryogen vessel to a gas exit path; and a controller (30) arranged to control the valve. The valve is arranged such that a gas pressure in the cryogen vessel exceeding a gas pressure in the gas exit path acts on the valve to open the valve and allow venting of cryogen gas. The valve is also arranged such that a gas pressure in the cryogen vessel inferior to a gas pressure in the gas exit path acts on the valve to urge it closed, so restricting flow of gas into the cryogen vessel.